Why is it colder at higher elevations? A thorough and visual explanation.
The Internet should be a place where we can find good answers to basic scientific questions. Unfortunately, that’s not always easy.
Consider the question, “Why is it colder at higher elevations?” It seems like you should be able to Google this and find a straightforward and enlightening answer. However, most online explanations (e.g. on question-and-answer websites like Quora, StackExchange, Reddit, and even many educational sites like Scientific American) are wishy-washy, incomplete, visually bankrupt, needlessly arcane, and/or grammatically shameful.
On Quora, a thread addressing the question of “Why is it colder at high elevation” contains 50 different user-generated answers, ranging from unsatisfying to spectacularly wrong. One Quora user, “Tzvi Freeman,” notes that reading through those answers gave him a feeling of “utter confusion and exasperation.” He continues:
“I searched for a simple answer to a kid’s question throughout the web and found so many different answers, very few of them making reference to the other answers. What am I supposed to tell the kid?”
This post is for you, Tzvi.
An instructive example: why is it so cold outside of an airplane?
Flying home from London this summer, I tapped the seat-back TV screen to check my flight’s real-time statistics. One noteworthy data point: it was negative 49°F outside.
The outside temperature had progressively decreased as we ascended up to 36,000 feet. Indeed, according to a longstanding scientific model known as the “International Standard Atmosphere,” one can expect air temperature to fall by roughly 3.5°F for every 1,000 feet of altitude gain in the “troposphere” (the layer of the atmosphere where people live and operate).
Say it’s 60°F where your plane lifts off. As you ascend to 35,000 feet, you can roughly expect the air temperature to drop by an astonishing 122°F. Outside your window at cruising altitude, the air would be an ungodly −62°F. (The temperature drop during my flight was slightly less severe, but that’s not surprising: variation from the the model is expected due to factors like time of year, weather, and geography.)
So what’s going on?
An explanation and a diagram
Here’s a simple but thorough sequence of events describing why it’s so cold outside an airplane:
- The sun radiates lots of sunlight, which carries lots of energy.
- The atmosphere is transparent to much of that incoming sunlight, especially visible light and some infrared light. This light passes through the air without being absorbed. As a result, air — the stuff outside a plane — is not directly heated by the sun.
- The earth, unlike the atmosphere, does readily absorb sunlight. Land and surface water soak up the sun, heat up (like a sidewalk on a summer day), and transfer warmth to air that’s touching the surface. Air is a poor conductor of heat, though, so the warmth it gets at ground level is not effectively relayed to higher elevations. Therefore, air near the ground will generally be the warmest.
- Air itself is constantly in motion; it will often rise toward the sky. But, any air that rises will expand and cool, as it encounters progressively lower atmospheric pressure (explained further in the next section). The upshot is that the air outside a plane will be much colder than at ground level.
Going deeper: why does rising air cool?
On the righthand side of the diagram above, a “parcel” of air rises toward the sky. The concept of an air parcel is helpful when analyzing how air behaves. An air parcel is a self-contained packet of a given amount air; it can change size, move in any direction, and have its own distinct temperature. (Since air moves quickly and is a poor conductor of heat, it’s reasonable to assume that an air parcel in motion will not exchange heat with its surroundings.)
There are many possible reasons why an air parcel may rise. For example, it may float upwards upon getting heated at the earth’s surface, since warm air is relatively buoyant. In other cases, a parcel may be forced upward when it runs into a mountain slope and has no place to go but up.
Any person, plane, or air parcel going “up” toward the sky will experience the same thing: as the altitude increases, the weight of the atmosphere pushing on you — the “atmospheric pressure” — decreases. That’s because as you ascend, there is progressively less air above you. This is visualized below using the analogy of a pancake stack.
When an air parcel rises, it will start to expand as it encounters less atmospheric pressure. This phenomenon is observable in everyday life. For example, if you take a bag of potato chips (which is basically an air parcel with some junk food inside) to a higher elevation, the air inside the bag will expand as the surrounding air pressure drops. The clip below shows a SunChips bag expanding as it’s driven up to Pikes Peak in Colorado (where air pressure is ~50% lower than you’d find sea level).
This same type of expansion is observable, even more dramatically, in a sturdy balloon. The time-lapse clip below shows a rising weather balloon, which approximates the behavior of an air parcel, expanding as the atmospheric pressure decreases during a ~90,000-foot ascent. The balloon expands to more than 100 times its original volume, until it finally says “no more” and explodes!
Rapid expansion of an air parcel (as it encounters lower atmospheric pressure) will cause it to cool significantly — generally a few degrees or more per 1,000 feet. The weather balloon above, for example, cooled to way below freezing temperature as it expanded.
This cooling occurs because, at the molecular level, an air parcel uses up some of its internal energy as it expands. In a sense, energy is required for the air parcel to “push out” into the environment. A reduction in internal energy corresponds to a reduction in heat energy. Therefore, when a gas’s internal energy decreases, so does its temperature. (If you are interested in the detailed thermodynamic behavior of gases and the nature of so-called “adiabatic expansion,” you can learn more here.)
A good household example of expansion-related cooling is letting air out of a bike tire. A guy named Ryan demonstrates this below on YouTube. Ryan lets air rush out of his bike tire; the air naturally expands as it moves from being under high pressure (inside the tire) to lower pressure (outside the tire). As expected, the air cools a lot in the process, as shown by the Celsius temperature gauge.
On the flip side, when an air parcel encounters greater atmospheric pressure, it will compress and warm. Ryan from above posted another video showing how the temperature of air increases when it is compressed to inflate a bike tire.
Similarly, an air parcel out in the wild gets compressed and warms when it moves from higher elevation to lower elevation. The increase in atmospheric pressure squishes the parcel, thereby transferring internal energy to it and increasing its temperature. That’s a major reason why Death Valley — the lowest point in North America — is so hot: any air parcel that descends to that low of an elevation undergoes intense compression and warming in the process.
A second instructive example: why is it colder in the mountains?
Mountains are colder than lower elevations for the same basic reason that it’s cold outside a plane: air is always on the move, and any air that moves upward in the atmosphere will expand and cool.
One major difference between planes and mountains is that when you’re on a mountain, you’re standing on land — rather than flying in the sky. Land can be quite effective at absorbing the sun’s energy and transferring warmth to nearby air. This type of warming doesn’t happen in the free atmosphere where a plane flies, since air itself does not readily absorb sunlight.
In this way, sunlight absorbed by the surface of a mountain or a high plateau will act to increase local temperatures. The bigger a mountain’s surface area, the larger the heating effect. However, there are reasons why mountains are still generally colder than land at lower elevations.
First, air is always on the move: a given mountain range — even if heated significantly by the sun — will encounter cold air blowing in from other sky-high locations. Much of this air will be quite chilly because it has not been warmed by a sun-baked surface. Indeed, the earth has a relatively small amount of high-elevation surface, and thus has limited ability to heat high-elevation air on a large scale.
Second, some air arriving in the mountains will have risen up from lower elevations, expanded, and cooled due to the decline in atmospheric pressure.
The highest peak in Hawaii offers a striking reminder of how temperatures can plummet as you go up a mountain. In just a 2-hour drive, you can venture from a balmy tropical beach to the snowcapped 14,000-foot summit of Mauna Kea (pictured below).
Addressing a couple other common questions
If hot air rises, then why is it colder at elevation?
It’s absolutely true that warmer air rises. An air parcel will keep rising as long as it’s warmer than the air surrounding it. This happens because warmer air is less dense, and thus more buoyant.
But we saw above how any air that rises will cool, due to encountering lower atmospheric pressure and expanding. The higher up an air parcel goes, the more its temperature will drop.
Therefore, in spite of the fact that warmer air rises, it cools quickly as it moves to higher elevations. What was once “warm” air has become definitively “cold.” Higher elevations are where we find this cold air.
You’re saying its colder the higher up you go, but then why is it warmer on the upper floor of my house?
Because your house has an air-trapping technology called a roof.
Warmer air rises because it’s relatively buoyant. If given the opportunity, it can rise to great heights and, as we’ve seen, cool down significantly in the process. But your roof impedes that process; warm air will pool at your ceiling instead.
Interestingly, air can be expected to cool slightly when rising from the lower level of your house to the upper level. That’s because the atmospheric pressure at the top of your house is slightly lower than at the bottom. (You can verify this with a barometer app on a smartphone.)
For example, imagine you have 80°F air parcels popping out of your heating vent on the ground floor; they rise to the top of the house because they are more buoyant than the rest of the air in your house (e.g. 70°F air). On a 25-foot journey upwards, the warm air parcels’ temperature can be expected to decrease about 0.2°F due to encountering lower pressure and expanding. The outcome is 79.8°F air hanging out at your ceiling.
In that case, you’ll have warm air sitting above cooler air. The technical term for such a situation is a temperature inversion. Inversions can and do occur at times in the actual atmosphere; if you become a weatherperson, you’ll learn all about them!
Conclusion
The next time you hike up a mountain, bring a jacket.
